At my company, I signed an agreement upon hiring that I would yield to a search of my vehicle in exchange for the right to park it on the company's property. I believe that, by doing so, I've voluntarily ceded any right to claim that car's contents are my private property (at least, with regard to searches by my employer).
My alternative was to seek different transportation, park offsite, or refuse to job.
If Weyerhauser's employees didn't sign a similar agreement (or were not informed of the policy), then I'd agree with your argument.
The agreement I signed says no firearms in their buildings and no CCW. I had them type out and enter into my contract a clause laying full responsibility for my safety on them for such situations where a firearm would be a reasonable deterrent for. Ie; a co-worker on a rampage. They hemmed an hawed, but signed it.
None of which applies to the above article though. While Weyerhauser may have changed their policy, there is no mention of the guy in question actually agreeing to the "No Firearms in your cars policy". He even volunteered to allow them to search his vehicle after their dogs "hit" on his truck. He felt he had nothing to hide, including his legally owned firearms.